The present invention relates to a method for fabricating light emitting diodes to be used for display, transmission or the like.
Nowadays, light emitting diodes are widely used for optical communications, information display panels and so on. It is important that the light emitting diodes used for these purposes have a high luminance. The luminance, or the efficiency of such a light emitting diode depends on an internal quantum efficiency and an outward emission efficiency. Among these, the outward emission efficiency is largely influenced by the element structure. To improve the outward emission efficiency, a current blocking layer is formed directly below a bonding pad with a view to suppressing an ineffective emitted light that does not go outside due to blockage by the bonding pad or with a view to reducing an emitted light that is totally reflected on a surface of the light emitting diode.
One of known light emitting diodes designed to suppress the above-mentioned ineffective emitted light has a structure shown in FIG. 15. This light emitting diode is formed by stacking an n-type layer 62, a light emitting layer 63, a p-type layer 64, p-type current diffusing layers 65 and 66, and an electrode 68 on a front surface of an n-type GaAs semiconductor substrate 61, and another electrode 67 on a rear surface of the substrate 61. Further, an n-type current blocking layer 69 is formed in the p-type current diffusing layer 66 directly below the front electrode 68. This n-type current blocking layer 69 makes it difficult for a current to flow directly below the front electrode 68, thereby suppressing generation of the ineffective emitted light that is hindered from going outside by the front electrode 68.
On the other hand, a known method for reducing the emitted light that is totally reflected adopts a technique of roughing the surface of the light emitting diode or forming a mesa-shaped portion on the surface of the light emitting diode so as to allow light to be emitted only from directly below the mesa-shaped portion. Examples of the light emitting diode formed with such a mesa-shaped portion are shown in FIG. 16 and FIG. 17. The light emitting diode shown in FIG. 16 is constructed of an n-type layer 71, a light emitting layer 72, a p-type layer 73, a p-type current diffusing layer 74, an n-type current blocking layer 75, a mesa-shaped p-type cladding layer 76, a p-type contact layer 77, a front electrode 78, and a rear electrode 79 which are formed on an n-type GaAs semiconductor substrate 70. The n-type current blocking layer 75 serves to limit the emission of light to a portion directly below the p-type cladding layer 76 to thereby improve the outward emission efficiency.
In regard to the light emitting diode of FIG. 17, the same components as those of the light emitting diode of FIG. 16 are denoted by the same reference numerals, and no description is provided for them. Similar to the light emitting diode of FIG. 16, this light emitting diode suppresses the ineffective, or invalid emitted light below the front electrode 78 by limiting the light emission to a portion directly below the mesa-shaped p-type cladding layer 76 defined by the current blocking layer 75 and forming the electrode 78 on the current blocking layer 75 in a position where no mesa-shaped p-type cladding layer 76 exists.
The light emitting diode of FIG. 15 has a problem that the total reflection of the emitted light is not suppressed. Furthermore, if a substrate with an inclined surface is used as the semiconductor substrate 61, assuming that the current diffusing layer 66 consists of an Al.sub.x Ga.sub.y In.sub.1-x-y P (0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1) film, then a shape reflecting the current blocking layer 69 appears on a surface of the film in a displaced position due to a slow growth rate on a (1 0 0) plane of the crystal. Then, assuming that the angle of inclination of the semiconductor substrate 61 is .theta. and that the thickness of the current diffusing layer 66 is d, then the amount of displacement becomes d/tan .theta.. Therefore, if the current diffusing layer 66 formed is thick, then the displacement of the upper electrode 68 from the current blocking layer 69 cannot be ignored, resulting in a reduced outward emission efficiency. For example, if the top surface of the semiconductor substrate 61 is inclined at an angle of 15.degree. in a [0 1 1] direction with respect to the (1 0 0) plane and the thickness of the current diffusing layer 66 is 7 .mu.m, then the difference in position between the current blocking layer 69 and the electrode 68 becomes 26 .mu.m. Therefore, considering the fact that the size of the electrode 68 is normally 100 .mu.m.phi. to 120 .mu.m.phi., the difference in position between the current blocking layer 69 and the electrode 68 is equivalent to about one fourth of the size of the electrode 68.
In the light emitting diode shown in FIG. 16, the p-type cladding layer (current diffusing layer) 76 has a mesa-like shape. Therefore, the total reflection of the emitted light on the surface is suppressed, whereas suppression of the ineffective emission under the front electrode 78 is not achieved. Furthermore, etching of the current blocking layer 75 and etching of the p-type cladding layer 76 for the formation of the mesa-like shape must be performed separately. Thus, the fabricating process becomes complicated.
In the light emitting diode shown in FIG. 17, both the total reflection of the emitted light and the ineffective emitted light under the front electrode 78 are suppressed. However, similarly to the light emitting diode of FIG. 16, etching of the current blocking layer 75 and etching of the p-type cladding layer 76 for the formation of the mesa-like shape must be performed separately, and this leads to the problem that the fabricating process becomes complicated.